1. Field of the Invention
The present invention relates to an aggregate dispersant, a method of manufacturing an aggregate of resin-containing particles, a toner, a developer, a developing apparatus, and an image forming apparatus.
2. Description of the Related Art
A toner which develops a latent image is used for a variety of image forming processes, and as an example of use of the toner is known a use in an electrophotographic image forming process. An image forming apparatus which forms images in an electrophotographic system includes a photoreceptor, a charging section, an exposing section, a developing section, a transfer section, and a fixing section. The charging section charges a surface of the photoreceptor. The exposing section irradiates the charged surface of the photoreceptor with signal light to thereby form an electrostatic latent image corresponding to image information. The developing section supplies a toner contained in a developer to the electrostatic latent image formed on the surface of the photoreceptor so that a toner image is formed. The transfer section transfers the toner image formed on the surface of the photoreceptor to a recording medium. The fixing section fixes the transferred toner image onto the recording medium. The cleaning section cleans the surface of the photoreceptor from which the toner image has been transferred. In the image forming apparatus as described above, the electrostatic latent image is developed by using as the developer a one-component developer containing a toner or a two-component developer containing toner and carrier so that an image is formed. The toner used in the above case is formed of resin particles which are obtained in a manner that, for example, colorant and a release agent such as wax are dispersed and granulated in binder resin serving as a matrix.
Through the electrophotographic image forming apparatus, an image having favorable image quality can be formed at high speed and low cost. This promotes the use of the electrophotographic image forming apparatus in a copier, a printer, a facsimile, or the like machine, resulting in a remarkable spread thereof in recent years. Simultaneously, the image forming apparatus has faced up to more demanding requirements. Among such requirements, particular attentions are directed to enhancement in definition and resolution, stabilization of image quality, and an increase in image forming speed, regarding an image being formed by the image forming apparatus. In order to fulfill these demands, a two-way approach is indispensable in view of both the image forming process and the developer.
Regarding the enhancement in definition and resolution of the image, the reduction in diameter of toner particles is one of problems to be solved from the aspect of the developer. This is based on the perspective such that it is important to authentically reproduce the electrostatic latent image. As a method of manufacturing the diameter-reduced toner particles, the emulsion aggregation method is known, for example. In the emulsion aggregation method, coloring resin particles containing binder resin, colorant, a release agent, and the like ingredient are generated and then aggregated in water, thus manufacturing toner particles.
As the diameter-reduced toner particles manufactured by the emulsion aggregation method, there is a toner which exhibits an acid number falling in a range of 1.0 mg KOH/g to 20 mg KOH/g and contains 3% by weight or less of a residual surfactant in the toner particles and 10 ppm or more and 1% by weight or less of an aggregating agent composed of salt of divalent or higher valent water-soluble inorganic metal having charges (refer to Japanese Examined Patent Publication JP-B2 3107062, for example).
The toner disclosed in JP-B2 3107062 is manufactured as follows. First of all, resin fine particle dispersion, colorant dispersion, and wax dispersion were mixed with each to obtain an admixture. To a dispersion medium of the admixture is then added the aggregating agent dispersible therein, which contains at least the salt of divalent or higher valent inorganic metal having charges, thereby forming aggregates. The aggregates are then heated up to a temperature equal to or higher than a glass transition temperature of the resin so that the aggregates are fused, thus resulting in toner particles. In a manufacturing method as just described, an amount of the surfactant contained in the toner particles is set at a predetermined level or lower, the content of the salt of divalent or higher valent inorganic metal used for aggregation is set to fall in a specific range, and an ion bridge is introduced into binder resin. The toner disclosed in JP-B2 3107062 is thus obtained.
In the method of manufacturing a toner as stated above, the resin fine particle dispersion composed of an aqueous medium and resin particles formed therein, the colorant dispersion composed of an aqueous medium and colorant particles formed therein, and the wax dispersion composed of an aqueous medium and wax particles formed therein are mixed with each other, thereby aggregating the resin particles, the colorant particles, and the wax particles. This leads to formation of a toner which is an aggregate of respective particles. Such a toner in form of aggregate of particles has pigment particles and wax particles exposed on a surface of the toner. The exposure of the wax particles on the surface of the toner will cause a decrease in preservation stability and further, when the wax particles are detached from the toner, the anti-offset property will deteriorate. In addition, the exposure of the colorant particles will cause the toner to exhibit nonuniform charging performance. Moreover, the variation among ratios of the resin, colorant, and wax contained in the respective aggregates may result in a failure to benefit the charging stability of the toner.
In view of the problem as described above, there has been proposed another method of obtaining a toner formed of aggregates of respective particles (refer to Japanese Unexamined Patent Publication JP-A 2004-295028, for example). In the method, low-molecular-weight resin incorporates 25% by weight to 75% by weight of wax and colorant to thereby form wax masterbatch particles which are then aggregated as well as binder resin particles. According to JP-A 2004-295028, the wax masterbatch is prepared by incorporating the wax and colorant into the low-molecular-weight resin and then treated with a dry or wet pulverization, thereby forming 10 nm to 5 mm-sized wax masterbatch particles. Furthermore, the binder resin particles whose average particle diameter falls in a range from 50 nm to 800 nm are prepared by emulsion polymerization. Subsequently, dispersion in which the formed wax masterbatch particles are dispersed is mixed with dispersion in which the formed binder resin particles are dispersed so that the wax masterbatch particles and the binder resin particles are aggregated. The aggregates of particles are then heated to be fused with each other. A toner is thus formed.
In the toner disclosed in JP-A 2004-295028, the wax masterbatch particles contain the wax and colorant in a dispersed state, which are smaller in particle diameter than the wax masterbatch particles. Amounts of the pigment and wax can be thus decreased which are exposed on the surface of the aggregate composed of aggregated wax masterbatch particles and binder resin particles described above, as compared to those in the toner disclosed in JP-B2 3107062.
The toner disclosed in JP-A 2004-295028 may, however, suffer from the variation in respective contents of colorant, wax, etc. in the toner because the toner is formed of the wax masterbatch particles and the binder resin particles, that is to say, the toner is formed of aggregated particles which are different in component and composition. As a result, the problem of failing to benefit the charging stability is not solved even by the toner disclosed in JP-A 2004-295028. Furthermore, the toner disclosed in JP-A 2004-295028 requires respective fabrications of the wax masterbatch particles and the binder resin particles, which makes the manufacturing process complicated.